1. Field of the Invention
The invention relates to a method of wire-bonding for bonding electrodes of a semiconductor device to associated lead pins through a wire, and more particularly to a method of determining an order of wire-bonding.
2. Description of the Related Art
Although a semiconductor device is usually formed with electrodes along a periphery thereof, some semiconductor devices are formed with electrodes 2 longitudinally in a line at the center of a device 1, as illustrated in FIG. 1. Such a device as illustrated in FIG. 1 can be found as a dynamic semiconductor memory.
Each of electrodes of a semiconductor device is to be connected to a lead pin associated therewith through a wire. In the device 1 illustrated in FIG. 1 having the longitudinally, linearly arranged electrodes 2 at the center thereof, lead pins 3 are arranged on a surface of the semiconductor device 1 by means of a double-sided adhesive tape 9, and then connected to the electrodes 2 through a wire 4.
In wire-bonding for connecting each of the electrodes 2 to the associated lead 3 through the wire 4, at a tip end of the wire 4 supported by a capillary 6 is formed a small metal ball 5, for instance, by electric discharge, as illustrated in FIG. 2A. Then, as illustrated in FIG. 2B, the capillary 6 is lowered to thereby compress and hence bond the metal ball 5 onto the electrode 2. Then, the capillary 6 is raised, as illustrated in FIG. 2C. Then, as illustrated in FIG. 2D, the capillary is made to horizontally move away from the lead 3 to which the wire 4 supported by the capillary 6 is to be connected. Since the distal end of the wire 4 is bonded to the electrode 2, there is formed a bending portion 7 in the wire 4 as the capillary 6 moves. The formation of the bending portion 7 advantageously makes uniform the shape of the wires 4 having been bonded to the lead 3, and in particular important for making maximum height of the wire 4 measured from a surface of the semiconductor device 1 uniform.
After the formation of the bending portion 7 of the wire 4, the capillary 6 is made to move towards the lead 3 as the wire 4 is drawn out through the capillary 6, as illustrated in FIG. 2E. When the capillary 6 reaches above the lead 3, the capillary 6 is lowered to compress the wire onto the lead 3 to thereby bond the wire 4 to the lead 3, as illustrated in FIG. 2F.
The above mentioned steps are carried out for bonding the single electrode 2 to the single lead 3 associated therewith through the wire 4. The semiconductor device 1 usually has a plurality of the electrodes 2 and the leads 3. In the device 1 illustrated in FIG. 1, there has to be carried out wire-bonding 26 times, namely, by the number of times equal to the number of the electrodes 2, except so-called non-connect electrodes.
An order of wire-bonding in such 26 times wire-bonding is determined as shown in FIGS. 3A to 3F. In brief, wire-bonding is carried out in accordance with an order at which electrodes are disposed. Namely, a firstly disposed electrode 2-1 located in alignment with a lead 3-1 located at an end of a lead array 3A is first bonded to the lead 3-1 through a wire 4-1, as illustrated in FIG. 3A. Then, a secondly disposed electrode 2-2 located in alignment with a lead 3-2 located at an end of a lead array 3B is secondly bonded to the lead 3-2 through a wire 4-2, as illustrated in FIG. 3B. Then, a thirdly disposed electrode 2-3 is thirdly bonded through a wire 4-3 to a lead 3-3 located adjacent to the lead 3-1, as illustrated in FIG. 3C. In the same way, electrodes 2-4 to 2-26 (not numbered) are successively bonded to leads 3-4 to 3-26 (not numbered) through wires 4-4 to 4-26 (not numbered), as illustrated in FIGS. 3D to 3F.
As having been explained with reference to FIGS. 2A to 2F, in order to uniformalize the shape of the bonded wires 4, the following steps are required: (a) bonding the metal wire ball 5 attached at a tip end of the wire 4 to the electrode 2 disposed on the semiconductor device 1, (b) raising the capillary above the electrode 2, and (c) horizontally moving the wire 4 away from the lead 3 to thereby form the bending portion 7 of the wire 4.
However, as illustrated in FIG. 3D, for instance, before a wire 4-13 is bonded to an electrode 2-13, a wire 4-12 has already been bonded between an electrode 2-12 and a lead 3-12. The capillary has 6 to be moved toward the already bonded wire 4-12 for wire-bonding of the wire 4-13, and hence, the capillary 6 may unpreferably contact the wire 4-12 to thereby deform or break the wire 4-12.
For resolving such a problem, the location of the electrodes 2 and the leads 3 is limited, which brings difficulty in design of a semiconductor device and leads. In addition, it is necessary to prepare additional area in which a capillary can move without obstruction. Such additional area poses a problem of the semiconductor being larger in size.
U.S. Pat. No. 5,269,452 issued to Sterczyk on Dec. 14, 1993 has suggested another method of wire-bonding. The suggested method of bonding each of chip pads of multiple pins to a lead frame includes the step of firstly carrying out wire-bonding for chip pads located at corners of a semiconductor device, and then carrying out wire-bonding for chip pads located at the center of a semiconductor device. These two steps are repeated. An advantage obtained by the method is that an already completed wire does not contact and hence is not deformed by a wire-bonding tool.